This invention relates to a writing instrument having a radially displaceable grip member, the inner side of which is filled with a fluid, provided at a part of the body of the writing instrument which is held by the fingers of a person when it is used for writing, and having an axially movable member which moves in accordance with an increase and a decrease of a fluid pressure caused by depressing or releasing the grip member during writing. More particularly, in the case where the writing instrument is a mechanical pencil, lead can be extended, clamped and released without changing the positions of the fingers holding the writing instrument or without any special action to the instrument during writing, so that the lead can be substantially automatically extended or retracted. Further, in the case where the writing instrument is a marking pen or a ballpoint pen, using ink, a writing tip of the instrument is always maintained positionally inside of the body when it is not in use. and the writing tip is advanced for writing by a gripping force applied to the body, so that the problem of the writing tip staining a pocket or a cloth can be eliminated, and further the writing tip is protected from being deformed.
In the conventional mechanical pencils, there is known a so-called "automatic lead advancing and retracting type.revreaction. having an intricate lead gripping mechanism which allows the lead to move forwardly but not rearwardly. In this type, lead is protruded by a sliding member which is always urged forwardly.
Further, there i known another type of the mechanical pencil, a so-called "gripping type", which can ensure an effect similar to that of the automatic lead advancing and retracting type. A gripping member is provided where the operator finger grips the body during writing, and the lead is clamped, protruded and released by a mechanism in which a radial displacement of the grip member is translated into an axial displacement of a writing member having the writing tip.
In the conventional writing instrument such as a marking pen or a ballpoint pen, using an aqueous ink, a writing tip of the instrument projects from an end of the body by pushing or turning the mechanism when it is used for writing. The writing tip is retracted into the body by pushing or turning the mechanism when it is not in use, so that a problem of the writing tip staining a pocket or cloth can be prevented.
There is another type of the conventional writing instrument having a clip mechanism by which the writing tip is retracted into the body when it is not in use, or having a writing tip retracting mechanism operated by pushing a part of the body, while the writing tip is projected by pushing the mechanism.
All of these conventional mechanism, however, have the following disadvantages:
The automatic lead advancing and retracting mechanism is intricate and expensive. A stable writing operation can not be ensured because additional writing force against the forward force of the sliding member is required, and further, the lead would not project, during writing, from a lead pipe provided at a frontmost end of the body.
Further, although the gripping type of mechanical pencil has a simple construction and assembly is not difficult, the mechanical pencil is, however, undesirable from a design standpoint in view of the appearance and outside dimensions due to the provision of the grip member. In the gripping type, it may be difficult to turn the body with the fingers during writing to thereby permit a sharp-shaped portion of the tip end of the lead to project onto a paper or the like, and, further, it may be difficult to ensure a sufficient axial stroke of the grip member which translates a radial displacement of the grip member to an axial force allowing an axial movable member to move.
With respect to a writing instrument in which the writing tip is retracted into the body by pushing or turning the mechanism, if one fails to push or turn the mechanism and puts the pen in his pocket, the ink is likely to stain his clothes.
As for the writing instrument utilizing the clip mechanism, the writing tip can be retracted into the body only if the clip is inserted over the flap of a pocket. This safety mechanism, however, does not work if the entire instrument is put in the pocket. Moreover, there may be a problem that the writing tip touches and stains the clothes before the insertion of the clip and the actuation of the safety mechanism responding to its movement.
Furthermore, in the writing instrument having a mechanism which is pushed or turned to protect the writing tip from being deformed, if one fails to push or turn the mechanism and puts the instrument in his pocket, the writing tip may be deformed.
The sliding-type mechanical pencil or a double-pushing type mechanical pencil requires additional intricate operation to retract the writing tip into the body.